The techniques employed for the application of wax and lipid coatings on solid dosage forms include the hot-melt application whereby the waxes and lipids are melted with an oil-soluble emulsifying agent and are applied to tablets or granules rotating in a coating pan.
Another method in use employs organic solvent solutions of waxy materials in which the waxes and lipids are dissolved in a suitable organic solvent such as chloroform and the organic solution is then applied on tablets or pellets in a conventional coating pan, for example.
Spray congealing is also in use and presents the advantage of producing directly a free flowing powder by spraying a suspension of finely-divided powdered medicament, molten wax and lipid into solid particulate form.
Aqueous film coating has gained use recently. A typical process involves dispersing a finely-divided powder of hydroxypropylmethylcellulose phthalate in an aqueous medium containing triacetin to give an aqueous coating liquid, spraying the coating onto the surface of a solid dosage form and thereafter drying the thus coated solid dosage form to provide an enteric coating.
None of these processes have been completely satisfactory. Organic coating liquids are undesirable as the use of large amounts of organic solvents involves problems of explosion or fire hazard as well as possible toxic effects and environmental pollution. The hot-melt and spray congealing methods involve the use of special equipment which is costly.
The use of aqueous polymer solutions for film coating purposes is limited to relatively low solids contents since the viscosity of such solutions rises sharply with an increase in concentration or molecular weight of the polymer. As a consequence of this relatively low polymer concentration, a number of separate layers of polymer must be built up in order to obtain a coating of adequate thickness for surface protection. This, in addition to the slow rate of evaporation of water and relatively large amounts of water to be removed, may result in a very long processing time. Furthermore, water sensitive drugs are prone to hydrolysis due to the long exposure of the drugs to water and many tablets must be sealed with water barriers such as shellac coatings prior to aqueous film coating.
It is an object of the present invention to provide a novel aqueous coating system which has none of the problems associated with the methods currently in use by the pharmaceutical industry.
It is a further object of the present invention to provide a novel aqueous coating system which can be used in current coating equipment without requiring the use of special expensive equipment or volatile solvents as in the case of the prior art methods presently in use.